Using Raman scattering, the quasi-two-dimensional organic superconductor $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{Cu}\left[{\mathrm{N}\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$ ($T_c=11.8$ K) and the related antiferromagnet $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{Cu}\left[{\mathrm{N}\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$ are studied. Raman scattering provides unique spectroscopic information about magnetic degrees of freedom that has been otherwise unavailable on such organic conductors. Below $T=200$ K a broad band at about 500 ${\mathrm{cm}}^{-1}$ develops in both compounds. We identify this band with two-magnon excitation. The position and the temperature dependence of the spectral weight are similar in the antiferromagnet and in the metallic Fermi liquid. We conclude that antiferromagnetic correlations are similarly present in the magnetic insulator and the Fermi-liquid state of the superconductor.

• Received 21 July 2015
• Revised 1 October 2015

DOI:https://doi.org/10.1103/PhysRevB.92.161112